Ion homeostasis in the extracellular space

Maintenance of the extracellular ion composition is of paramount importance for brain function, because every shift in ion concentrations profoundly affects the membrane properties of nerve cells and hence their excitability. Brain extracellular space contains high amounts of Na+ ([Na+]o — 130 mM) and Cl- ([Cl-]o — 100 mM), whereas it is rather low in K+ ([K+]o — 2-2.5 mM). This is reversed inside the brain cells, as the cytosol of most of neurones and glia is rich in K+ ([K+] - 100-140 mM) and poor in Na+ ([Na+] -<10 mM). The intracellular chloride concentration is, as a rule, low in neurones ([Cl-]j — 2-10 mM) and is relatively high in glial cells ([Cl-]j — 30-35 mM), due to Cl- influx in exchange for Na+ and K+ by the Na+/K+/Cl- transporters. The extracellular concentration of another important cation, Ca2+, is relatively low in the extracellular space ([Ca2+]o — 1.5-2 mM), nevertheless it is still about 20 000 times lower in the cytosol ([Ca2+]j ~ 0.0001 mM). These transmembrane ion gradients together with selective plasmalemmal ion channels form the basis for generation and maintenance of the resting membrane potential and underlie neuronal excitability. The safeguarding of transmembrane ion gradients is the task for numerous ion-transporting systems, which allow ion movements either by diffusion (ion channels) or at the expense of energy (ion pumps and exchangers). The main ion transporters operative in astrocytes are summarized in Figure 7.7.

Apart from preserving their own transmembrane ion homeostasis, astroglial cells are heavily involved in maintenance of extracellular ion concentrations. As neuronal activity is inevitably associated with influx of Na+ and Ca2+ (depolarization) and efflux of K+ (repolarization), the extracellular concentrations of these ions vary; the relative variations are especially high for K+ ions (due to a low [K+]o and a very limited volume of the CNS extracellular space; i.e. even relatively modest numbers of K+ ions released by neurones may very substantially affect the extracellular K+ concentration). If the K+ released during action potential propagation was allowed to accumulate in the extracellular space, this would cause neuronal depolarization. Small increases in [K+]o would increase neuronal excitability by bringing their membrane potential closer to the action potential threshold. If K+ rises sufficiently to depolarize the neurones past the threshold

Figure 7.7 Ion transporting systems in the astroglial cell. The main astroglial ion transporters are: Na+/H+ antiporter; Na+/H+/HCO- and Na+/K+/2Cl- cotransporters; Cl-/HCO-and Na+ /Ca2+ exchangers; Na+/K+ pumps; water channels (aquaporins); K+ and Cl- channels and swell-activated Cl- channels. Glia also contain carbonic anhydrase, which catalyzes the conversion of H2O + CO2 to carbonic acid (H2CO3), which readily dissociates to H+ and HCO-, hence facilitating CO2 uptake and extracellular pH regulation during neuronal activity

Presynaptic terminal

NaVCa'" exchanger

NaVCa'" exchanger

Ion Pump Homeostasis

Figure 7.7 Ion transporting systems in the astroglial cell. The main astroglial ion transporters are: Na+/H+ antiporter; Na+/H+/HCO- and Na+/K+/2Cl- cotransporters; Cl-/HCO-and Na+ /Ca2+ exchangers; Na+/K+ pumps; water channels (aquaporins); K+ and Cl- channels and swell-activated Cl- channels. Glia also contain carbonic anhydrase, which catalyzes the conversion of H2O + CO2 to carbonic acid (H2CO3), which readily dissociates to H+ and HCO-, hence facilitating CO2 uptake and extracellular pH regulation during neuronal activity

Blood vessel for Na+ channel activation, neurones become inexcitable because Na+ channels become inactivated and there is a conduction block. Astrocytes help prevent the accumulation of extracellular K+, thereby stabilizing neuronal activity.

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